This invention relates generally to systems and method of transforming color gamut from one color space to another and, more particularly, to a system and method which transforms color gamut continuously from the saturated outer shell to the desaturated inner area of the color space.
RGB displays increase saturation by increasing light intensity, while printers increase saturation by adding more pigment, which absorbs light. This means that RGB display color becomes lighter as it saturates while a printer color becomes darker as it saturates. Thus the saturated edges of a display gamut (e.g., sRGB display) tend to extend outside and above a printer""s gamut, while a printer""s gamut tends to extend below and outside the display""s gamut, resulting in the loss of some colors. Images in display space are rendered best with some sort of objective mapping, but objective mapping does not preserve saturation at the edges of the gamut white to full saturation (everything gets lighter and desaturated), resulting in the distortion of some other colors. Conversely, graphics edge colors are best preserved with a more direct mapping while merely preserving hue (i.e., the most saturated red on the display is mapped to the most saturated red of the same hue on the printer), but this oversaturates colors like skin tones.
It would be desirable to map a color display space into a printer space in such a way, as to map the colors in the center of the gamut (colors normally associated with photos, such as skin tones) to have an xe2x80x9cobjectivexe2x80x9d (or xe2x80x9cexactxe2x80x9d) mapping. This is so color photos appear the same on both the color display and the printed page. It would also be desirable to preserve the saturation of the colors on the edge of the gamut, which are usually used for graphics. Many mapping solutions use one transformation table for images and another transformation table for graphics.
Having two tables can be both time and resource expensive. Also, some images may contain both image elements and graphical elements, making use of either table on its own unsatisfactory. U.S. Pat. No. 5,734,802 to Maltz et al., xe2x80x9cBlended Look-up Table for Printing Images with Both Pictorial and Graphical Elementsxe2x80x9d proposes a morph of two separate tables. Trying to meld both of these aims into one color table can cause an abrupt discontinuity during the transition between the saturated outer shell of the color space and the desaturated inner area. It would be desirable to create a smooth and continuous mapping that transforms a display color space to a printer color space which would give saturated colors for graphics, realistic colors for photos, and smooth transitions between the two.
A method for transforming color gamut from a first color space to a second color space, includes dividing the first color space into a first plurality of regions in an objective space; dividing a subset of the second color space into a second plurality of regions in the objective space; mapping a first region of said first plurality into a first region of said second plurality substantially without change; mapping a second region of said first plurality comprising an upper gamut of the first color space into a complementary second region of said second plurality; and mapping a third region of said first plurality comprising a lower gamut of the first color space with a gamma correction into a complementary third region of said second plurality. The upper gamut may include white to full color and the lower gamut may include full color to black. The gamma correction is applied to an outer edge of the first color space for increasing lightness and saturation at the edge. The method may be used in an image forming device to generate mappings in real-time. Alternatively, the method may be used to generate a lookup table, which may be stored in memory. The objective space may be Lab space, Luv space, Lch space, Xyz space, etc. The color spaces may be divided and mapped such that the borders between the regions in the space provide that transitions in gamut are mathematically continuous. This provides a smooth and continuous mapping that can be used to transform a display color space to a printer color space, which would give saturated colors for graphics, realistic colors for photos, and smooth transitions between the two.
Apparatus for transforming a digital color image comprising a plurality of first data elements, from a first color space to a second color space, includes a memory storing a lookup table comprising a mapping of first data values in the first color space to second data values in the second color space, the lookup table being generated by: dividing the first color space into a first plurality of regions in an objective space; dividing a subset of the second color space into a second plurality of regions in the objective space; mapping a first region of said first plurality into a first region of said second plurality substantially without change; mapping a second region of said first plurality comprising an upper gamut of the first color space into a complementary second region of said second plurality; and mapping a third region of said first plurality comprising a lower gamut of the first color space with a gamma correction into a complementary third region of said second plurality; and a processor, responsive to the lookup table and for each first data element having a first data value, for locating a corresponding second data value.
The method may be used to transform images in RGB display space through an objective space (such as Lab space) into CMYK printer space. The method divides the RGB display space into at least three separate areas using vector math, and remaps those areas linearly into the same number of complementary areas in the printer""s color space. The first area represents colors in the center of the gamut and can be called a xe2x80x9cSafexe2x80x9d area, because this area is preserved without change. Colors in this region map substantially identically from RGB space to CMYK space, without distortion or loss of colors. The second area is the upper gamut of the RGB color space (including white to full color), which is mapped into a complementary area on the printer space (which may or may not include the full saturation at that hue leaf). The third area is the lower gamut (full to black), which is mapped with a gamma correction on the outer edge vector to increase lightness and saturation at the edge of the gamut.
The method and apparatus map a color display space into a printer space, for example, in such a way, as to map the colors in the center of the gamut (colors normally associated with photos, such as skin tones) to have an xe2x80x9cobjectivexe2x80x9d (or xe2x80x9cexactxe2x80x9d) mapping. Color photos generally appear the same on both the color display and the printed page in this region when an objective mapping is used. The method and apparatus also preserve the saturation of the colors on the edge of the gamut, which are usually used for graphics. The method and apparatus also create a smooth and continuous mapping that transforms a display color space to a printer color space, giving saturated colors for graphics, realistic colors for photos, and smooth transitions between the two.